The first point to note is that the Archbishop is not being asked to do the impossible. Ever since Archbishop Charles Longley invited Anglican bishops to the first Lambeth Conference in 1867 it has been accepted that it is for the Archbishop of Canterbury to decide which bishops should be invited. He can invite who he likes and not invite who he likes and he is not obliged to have the agreement of any other person or body about the matter. The buck stops with the Archbishop.
This means that Archbishop Welby can fulfil the requests made in both the bullet points in the GAFCON letter. However, this still leaves the question of whether he should do so. To answer this question it is necessary to recall what has taken place in the Anglican Communion in the twenty years since the Lambeth Conference of 1998.
Two key things have happened.
First, in spite of being repeatedly urged not to do so, a number of provinces of the Anglican Communion (The Episcopal Church, the Anglican Church in Canada, the Episcopal Church in Brazil, the Scottish Episcopal Church and the Anglican Church in Aorateara, New Zealand and Polynesia) have acted in ways that go against Scripture and Resolution 1.10 of the 1998 Lambeth Conference by accepting, in terms of both doctrine and practice, the blessing of same-sex sexual relationships, same-sex marriages and the ordination of those in same-sex sexual relationships.
Secondly, in response to these developments, Anglicans in the United States, Canada and Brazil who have remained loyal to Scripture and Lambeth 1.10 have established the two alternative orthodox provinces mentioned in the first bullet point– the Anglican Church in North America and the Anglican Church in Brazil.
By acting in the way that they have, those Anglican provinces which have accepted same-sex sexual relationships have rejected the obligations that go with being a member of the Anglican Communion.